We have failed to detect superoxide formation in lung slices and homogenates incubated with and without paraquat and with d-tryptophan, 1-tryptophan, or d,1-tryptophan. These experimnts were inspired by the publications of 0. Hayaishi (J. Biochem. (Tokyo), 79: 13p-21p,1976 and others) on the properties of indoleamin 2,3-dioxygenase, an enzyme present in high concentrations in many tissues, especially the intestines and lung, which requires superoxide anion as a catalyst. Lung autoradiographs from mice killed at 3, 24 and 48 hr after i.v. injection of tritium-labeled paraquat are in preparation. It is hoped that the high resolution obtainable with tritium-labeled paraquat will permit some conclusions about whether paraquat is picked up preferentially into certain cells.